Ocean Currents, Part V: Ice Cubes Demonstration


We have learned in the past few weeks that salt water is more dense than fresh water and that cold water is more dense than warm. How do the factors of salinity and temperature combine?
The two jars react very differently when the colored ice cubes are added to them.
We have learned in the past few weeks that salt water is more dense than fresh water and that cold water is more dense than warm. How do these factors, salinity and temperature, combine?
Before hand, make some colored ice cubes by adding food coloring to the water in an ice cube tray. We did this the night before.
Fill two identical jars about 3/4 full with tap water. Add about 1/4 cup Kosher salt to one of the jars, mix thoroughly and let sit. Can your students tell which one of them has salt water and one fresh water (without tasting)? Very carefully add 3-4 of the colored ice cubes to each jar. Add the same amount to each jar. Do not bump or disturb the jars.

Where is the colored water going? Which jar is the ice melting faster? What would this indicate?

Can you see a current flowing toward the bottom as the icy water carries the food coloring down with it as it sinks? As the temperature evens out, the food coloring mixes throughout the jar. This is the fresh water jar. The icy water is denser than the room temperature water and sinks. The sinking of the icy water helped set up currents in the jar which quickly mixed the icy water and the room temperature water.

In the other jar the ice melted more slowly and the food coloring formed a layer at the surface. Because the ice cubes were made of fresh water, as they melted, the water floated at the top of the denser salt water.

To demonstrate for the little boys how the salt-water could be more dense than fresh water, I took a cup with some marbles in it and told them that this is just like the water molecules and then added some salt to the cup. This is like the salt molecules, which are able to fit in between the water molecules, making the entire substance more dense.
The densest water in the ocean is formed around the Antarctica because the water is very cold and very salty. This combination causes it to become very dense and sink to the bottom of the ocean basin surrounding Antarctica. This water then travels north as the densest water in the ocean.
Sources::
GEMS: Ocean Currents
Grades 5-8
7 Activities
Students gain fascinating insights into our ocean planet through these innovative activities. They learn how wind, temperature, salinity, and density set water into motion, and they make an “in-depth” investigation of the key physical science concept of density. They model how pollution dumped in one location can spread throughout the ocean. Learning is placed in a real-world context as students predict and analyze routes taken by shipwrecked sailors, the 1990 Nike shoe spill, the raft Kon Tiki, and other voyages. In “Message in a Bottle,” students create stories to show what they've learned over the course of the unit.

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